This invention relates generally to the field of measuring and monitoring the characteristics of fluids and more particularly to a non-contact method and apparatus for monitoring the surface of a fluid using fiber optics which enables the determination of mechanical properties of fluids such as the surface tension.
There are a variety of methods for measurement of the surface tension of fluids which are known in the art. In the known Wilhelmy plate technique, a plate is placed in contact with the fluid surface, and is then withdrawn from the surface. The maximum force needed to break the plate away from the surface is measured to determine the fluid surface tension. This approach requires contact in the surface, and thereby disturbs the characteristics of the fluid and obstructs the flow of the fluid such that surface tension at the plate may not be the same as that away from the plate. In the case of thin films, the contact and removal of the plate may damage the film, and in the case of some rigid or viscous films, the plate may be forced away from the vertical, providing erroneous readings.
Another known approach to measurement of surface tension is the Langmuir balance technique which permits a differential measurement. In this technique, a float and flexible barrier divides a fluid surface into two regions: one region with a clean surface and one region with a surface covered with an insoluble film, the surface tension of which is to be measured. The float senses the differential force between the two regions. The float is coupled to a force measurement device permitting measurement of the average difference of the surface tension between the clean surface and film covered surface. This approach can only measure the gross average difference and can only measure surface tension at the boundary of the fluid, and thus can provide no information about inhomogeneity. In addition, the apparatus must be custom designed to fit a particular test fixture, and its position must be fixed. In addition, the Langmuir technique only measures a difference in surface tension, can only be used in uniaxial compression, and requires contact with the fluid, thereby introducing errors.
The capillary wave measurement technique can be used to determine surface tension without contacting the surface. This technique utilizes a capillary wave generator to produce capillary waves on the fluid surface. A beam of light is directed using lenses and mirrors onto the surface. The reflected light is then detected and analyzed to determine the surface tension of the fluid. Thus, this technique does not require contact with the film. However, the apparatus must be customized to accommodate the light sensing device and the lens/mirror structure needed to properly direct the light beams. Consequently, this method is difficult to implement, align, and use. In addition, such capillary wave instruments cannot be made easily portable.
Accordingly, it is an object of the invention to provide a novel non-contact method and apparatus for measurement of the surface tension of a fluid which employs the capillary wave technique, and which is portable and easy to use.
It is another object of the invention to provide a novel non-contact method and apparatus using interferometric detection for measurement of the surface tension of a fluid utilizing a fiber optic system.
It is another object of the invention to provide a novel non-contact method and apparatus using interferometric detection for measurement of the surface tension of a fluid which is compact and portable.
It is another object of the invention to provide a novel non-contact method and apparatus for monitoring the surface of a fluid using interferometric detection which does not require customizing to fit the test structure.
It is another object of the invention to provide a novel non-contact method and apparatus for monitoring the surface of a fluid using interferometric detection which is capable of monitoring and measuring a variety of fluid characteristics including surface tension, anisotropy of certain surface characteristics and changes in the elasticity of a surface film.
Briefly, according to one embodiment of the invention, apparatus and corresponding method are provided for monitoring the surface of a fluid. The apparatus comprises means for generating a surface wave on the surface of the fluid, a photo sensor for detecting optical energy, and optical means for generating a beam of optical energy, for example, a laser. A fiber optic coupling device coupled to the optical means directs a portion of the optical energy beam onto the surface of the fluid, another portion of the optical energy beam onto the photo sensor, and directs optical energy reflected from the surface of the fluid onto the photo detector. Means are provided to couple the photo detector to a phase sensitive detector to permit measurement of phase shifts indicative of surface tension changes and for determination of capillary wavelengths.